1. Field of the Invention
This invention relates to a method and apparatus for improving the separation efficiency of a cyclone. More particularly, it relates to an improved method and apparatus for separating solids from gases discharged from the overhead tube of a cyclone. The method and apparatus is especially useful under fluid catalytic cracking conditions and for retrofitting existing cyclones.
2. Discussion of the Prior Art
Cyclones are the most widely used type of equipment for separating solid particles from gases. In the cyclone, solid particle-laden gas enters a cylindrical or conical chamber tangentialy at one or more points and leaves through a central opening. The entrance to the cyclone is usually rectangular. The solid particles, by virtue of their inertia, tend to move toward an outside cyclone wall from which they are led into a bin at the bottom of the cyclone. A cyclone is essentially a settling chamber in which gravitational acceleration is replaced by centrifugal acceleration. At common operating conditions, the centrifugal separating force or acceleration may range from 5 times gravity in very large diameter, low-resistance cyclones, to 2500 times gravity in very small, high-resistance units.
Cyclones have been employed to remove solids and liquids from gases and solids from liquids, and have been operated at temperatures as high as 1000.degree. C. and pressures as high as 500 atmospheres. Cyclones generally remove solids from gases when particles of over 5 microns (0.0002 inches) outside diameter are involved, although some of the multiple-tube parallel units attain 80-85% efficiencies on particles of 3 micron outside diameter. In collecting particles of over 200 micron outside diameter, cyclones may be used, but gravity settling chambers are usually satisfactory. In special cases where the solid particles show a high degree of agglomeration, or where high solid particle concentrations (over 100 grams/cubic foot) are involved, cyclones will remove smaller solid particles. In certain cases, agglomeration results in efficiencies as high as 98% on solid particles having an ultimate particle size of 0.1 to 2 microns. Additional background on cyclones is available in Perry's Chemical Engineers' Handbook, Edited by Howard B. Crawford and Ross J. Kepler, 5th Edition, pages 20-81 to 20-87, McGraw-Hill (1973).
Cyclones are typically employed in fluid catalytic cracking (FCC) systems. Fluid catalytic cracking is the most important and widely used refinery process for converting heavy oils into more valuable gasoline and lighter products. Originally, cracking was accomplished thermally, but the catalytic process has almost completely replaced thermal cracking because more gasoline having a higher octane and less heavy oils are produced. The catalytic cracking processes in use today can be classified as either moving-bed or fluid-bed units. The fluid-bed units predominate and are termed fluid catalytic cracking. In fluid catalytic cracking, a hot oil feedstock is contacted with a catalyst in a riser. As the cracking reaction progresses, the catalyst is progressively deactivated by the formation of coke on the surface of the catalyst. The catalyst and hydrocarbon vapors are separated, and oil remaining on the catalyst is removed by steam stripping within a reactor vessel before the catalyst enters a regenerator vessel. The separated oil vapors are taken overhead to a fractionation tower for separation into streams having desired boiling ranges.
The catalyst leaving the fluid catalytic cracking reactor is termed "spent catalyst" and contains hydrocarbons which remain after stripping, and coke which adsorbs on its surface. In the regenerator, coke and any hydrocarbons are burned from the catalyst with air. The regenerator temperature and coke burnoff are controlled by varying the airflow rate. The cracking reaction is endothermic, and the regeneration reaction is exothermic. Typical fluidized catalytic cracking catalyst has an average particle size of 50 microns. Average reactor temperatures are above 950.degree. F., with feedstock temperatures from 600.degree. to 800.degree. F., and regenerator exit temperatures for catalyst from 1100.degree. to 1600.degree. F. Regenerator temperatures are carefully controlled to prevent catalyst deactivation by overheating. This is generally done by controlling the airflow to give a desired CO.sub.2 /CO ratio in the exit flue gases, as the burning to CO.sub.2 does not remove additional coke from the catalyst, compared to burning to CO, but only produces excess heat. Additional information on fluid catalytic cracking can be found in Petroleum Refining Technology and Economics, by James H. Gary and Glenn E. Handwerk, pages 86-113, Dekker (1975) and U.S. Pat. No. 4,219,407 to Haddad et al.
Fluid catalytic cracking reactors typically employ cyclones to separate solids from gases exiting a riser conversion zone. The cyclones may be arranged in an open or closed system. In an open system, the hydrocarbons from the riser pass into the reactor vessel atmosphere prior to passing out of the reactor vessel. In a closed system, the hydrocarbons predominantly pass through the cyclone system without venting to the reactor vessel atmosphere. Closed cyclone systems are further described in U.S. Pat. No. 4,502,947 to Haddad et al, which is incorporated herein by reference.
The cyclones in fluid catalytic cracking reactors are typically arranged in single or multi-stage cyclone systems to separate gaseous hydrocarbons from catalyst particles. In the single stage cyclone, gas and catalyst particles enter through an inlet duct. Some particles are separated from the gas and exit the bottom of the cyclone. Some particles remain with the gas. The gas and remaining entrained solids leave the top of the cyclone through a gas outlet tube.
In many applications, a single stage cyclone does not provide the separation efficiency required. For these cases, two or more cyclones are connected in series. With this design, the gas and entrained particles exiting from the gas outlet tube of the first cyclone stage are directed to the inlet of the second cyclone stage. This can be achieved by the use of ducting or by locating the first cyclone gas outlet tube in open communication with the second cyclone inlet.
These single and multi-stage cyclone systems have several deficiencies. As stated above, when a single cyclone stage does not provide the separation efficiency required, it becomes necessary to add stages. The addition of a second or third stage increases the cost of installation. In addition, cyclones are typically located within a reactor vessel so additional space is required to house the additional cyclones. In the case of a high temperature, high pressure process, this adds significantly to the cost.
Another drawback is that as the particles progress from one cyclone stage to the next it becomes necessary to increase the cyclone efficiency in order to handle the smaller particle sizes that escape collection in the previous cyclone. With the current art, pressure drop through the cyclone increases when collection is increased. Increasing collection efficiency also increases dipleg length and horsepower required to move the gas and particles through the cyclone system.
Another problem occurs in the case of an existing installation when it becomes necessary to increase cyclone separation efficiency. With the present art, the cyclone stage must be added in the limited space of an existing catalytic cracking reactor. This can involve costly modifications and a compromised design.
Devices have been attached to cyclones to improve their efficiency. U.S. Pat. No. 4,244,716 to Duske discloses a skimmer secured to the top of a cyclone separator. The skimmer includes an outer cylinder forming an extension of the conventional air exhaust duct of the separator, and an inner concentric cylinder terminating within the lower end of the outer cylinder. The skimmer employs a spiral along its wall to direct fine dust-laden particles through a discharge opening. The device also employs a door to open and close the discharge opening. The inner and outer cylinders define an annular space aligned with the peripheral layer of upwardly moving air from the cyclone.
The device disclosed in U.S. Pat. No. 4,244,716 has a number of disadvantages. The spiral located within the skimmer device and the door for its discharge opening would not tolerate the hostile temperature conditions of a fluid catalytic cracking reactor vessel (greater than 950.degree. F.). Furthermore, the device requires an inner cylinder which is concentric with the cyclone gas outlet tube, and has a narrower diameter than the cyclone gas outlet tube. This results in increased pressure drop for the cyclone gas outlet tube. Increasing pressure drop through the cyclone gas outlet tube decreases cyclone efficiency.